This proposal is to continue a series of single unit recordings from Purkinje cells in the pars intermedia and hemisphere of the cerebellar cortex that are related to arm movements or to smooth tracking. Several questions about the tracking signals represented in the discharge of Purkinje cells will be addressed. Specific aim 1 tests and extends the hypothesis that Purkinje cell discharge is tuned to a preferred velocity and direction. Purkinje cell simple spike and complex spike discharge will be evaluated while the animal generates manual, circular tracking movements. The possibility that a population code predicts movement velocity will be tested by analysis of population responses during more complex tracking tasks (spiral tracking). Specific aim 2 is to determine to what degree Purkinje cell discharge is predictive and to what degree it is dependent on visual feedback. This will be accomplished by brief removal of visual feedback during circular tracking. Specific aim 3 will examine the effects of unpredictable perturbations of target motion on the discharge of Purkinje cells. Animals will perform smooth tracking movements along a straight line, and the direction and speed of the visual target that prompts the movement will be perturbed. The fourth specific aim is to compare the correlation of Purkinje cell discharge with movement dynamics to the correlation with movement kinematics. An attempt will be made to maintain the kinematics of manual tracking constant while manipulating the inertial, viscous or elastic forces that are required to produce smooth tracking arm movements with a two-dimensional robot.